777F, 777G, and 785D OEM Off-Highway Truck/Tractors Water Delivery System Caterpillar

Illustration 1	g03665654
Rear of Engine (1) Hydraulic pump

The water delivery system hydraulic pump (1) is located on the rear of the engine.

Pump (1) is axial and variable displacement. The maximum displacement is 105 cc (6.41 cubic inch) per revolution. Pump charge pressure is 1902 kPa (276 psi) with a main relief pressure of 30000 kPa (4351 psi).

The pump is electrically controlled via the cab XA2-A2 module. A pulse-width modulation signal producing variable amperage controls the pump proportional displacement solenoid.

Illustration 2	g03665659
Bottom View of Piston Pump (water Delivery) (2) Port "F" (3) Pump solenoid (4) Combination valves (5) Port "L" (6) Port "Z"

The water delivery system hydraulic pump (1) is controlled by an electronic displacement control (EDC). The EDC is equipped with one proportional pump solenoid (3). The display prompts the XA2-A2 cab module to send a pulse-width signal to the pump solenoid to control oil flow to the WDS motor.

Pump (1) is equipped with two internal combination valves (4). Each combination valve contains a high- pressure relief function and a low-pressure makeup function. The relief valves are set to 30000 kPa (4351 psi). The relief function of these valves limits the pressure in the water pump hydraulic circuit. The makeup function of the valves allows charge oil to flow into the low-pressure circuit. Makeup oil replenishes the oil lost to internal leakage and loop flushing.

A charge relief valve is located under port "F" (2) to regulate the available charge pressure. The charge relief valve pressure is set to 1902 kPa (276 psi).

Illustration 3	g03665671
Rear View of Piston Pump (7) Port "S" (8) Port "P"

Except for oil redirected for case flushing and cooling, the system is closed-loop to the motor.

The pump solenoid controls the available charge pressure to the swashplate servo. The swashplate servo varies the swashplate angle in order to provide variable pump displacement. When the swashplate is at zero angle, the pump does not generate flow in the closed-loop circuit. When the commanded displacement is greater than zero, port "P" (8) outlet directs the oil flow to the WDS motor.

The internal leakage in the system is inherently low. Oil flow is primarily closed-loop from port "P" (8) of pump (1) to the motor and back into the pump through port "S" (7). The excess charge oil flow is relieved over a charge relief valve into the pump case drain. In this process helps cool the pump. This purged oil flows from port "L" (5) to the hydraulic oil cooler, providing a portion of the cooling flow to the system.

Several ports are located in the housing of pump (1). Each of these ports is identified by a lettered designation which is cast into the housing next to the port. The above illustration and the previous illustration show the location of these ports. The function of each port used in the water delivery piston pump system is as follows:

Port "S" (7) is the low-pressure port of the hydrostatic drive loop. Return oil from the motor enters port "S" when the system is operating.

Port "F" (2) is the charge inlet port. Filtered oil from the charge pump is available at this port when the engine is running.

Port "L" (5) is a case drain port. In the water delivery piston pump system, case drain oil flows from port "L" to the hydraulic cooler and hydraulic oil tank.

Port "Z" (6) is the control pressure outlet port to the WDS motor. The control pressure from port "Z" operates the control valves in the WDS motor.

Port "P" (8) is the high-pressure port of the hydrostatic drive loop. Oil flows from port "P" to the WDS motor when the pump is providing oil flow to the motor.